Hot-melt adhesives, e.g. polyamides, polyesters and copolymers thereof, can be used in a wide range of applications. The polar amide or ester groups provide excellent cohesive strength and adhesion to polar substrates like paper, wood and metal. Also partly because of the presence of the polar groups, the materials are solids which are easy to handle at room temperature, and melt at a well-defined temperature range to produce an easily applied fluid which solidifies quickly to produce the adhesive bond. An obvious benefit is the absence of any solvent, which makes hot melt adhesives a technology of increasing importance.
Unfortunately, known polyamide hot melt adhesives interact poorly with low-energy substrates such as polyolefins, resulting in poor adhesive strength. In addition, low-energy substrates tend to be flexible (e.g. polyolefin films) and are used under a wide variety of conditions, such as over a wide temperature range. This requires suitable adhesives to have good flexibility, for example at temperatures as low as −60° C., and an ability to absorb differences in thermal expansion of substrates. Known polyamide hot-melt adhesives tend to have insufficient flexibility.
It is this combination of low-temperature flexibility, and adhesion to low energy substrates, whilst maintaining cohesive strength that has been difficult to achieve with polyamide hot melt adhesives.